1. Field
The disclosed concept pertains generally to output apparatus and, more particularly, to such output apparatus to output a vital output.
2. Background Information
Vital control systems using multiple processors need a mechanism to vitally control single point logical outputs. This means that two vital processors need to independently force a vital output state to the most restrictive state regardless of the operation of the other vital processor. Therefore, both vital processors need to agree to put the vital output into the least restrictive state.
Both vital processors need to be able to monitor and verify the current vital output state.
Output control and monitoring circuits can fail so both vital processors need to be able to frequently verify that they can correctly detect the vital output state and independently force an output in the least restrictive state to the most restrictive state. Any single point failure of the circuit either needs to be detectable or force the vital output to the most restrictive state. Ideally, the verification functions should be done without changing the state of the vital output.
In the event that either or both vital processors cease to execute their control programs, then the vital output needs to switch to the most restrictive state.
There is room for improvement in output apparatus to output a vital output.